The Doctor’s line to the Mother in Death to 2021 is a reference to the BBC sci-fi show Doctor Who.

In the Netflix documentary Death to 2021, The Doctor tells the Mother that he has never had a friend quite like her. The quote comes from an episode of BBC’s sci-fi series Doctor Who, which features two versions of the same character played by Jodie Whittaker and Peter Capaldi. In this particular scene, The Doctor is talking to his best friend in his “dark days” after losing his wife and daughter. The line comes from “The Woman Who Fell To Earth” and has been used as a hashtag on Twitter in response to some of The Doctor’s more heart-wrenching moments.